2 options on SL for NFP
1 - If you are taking the trade at the time of the news with a buy/sell stop then use a trailing stop of 10 to 20 pips. (todays trade)
2 - If you are taking the trade after the time of news (15m, 30m 1H) you will see a new candle that will create a mini swing high/low close to price that you can put the SL behind.